Experience-dependent plasticity and spontaneous activity depend on cell identity in layer V of the barrel cortex.

Experience dependent plasticity (EDP) has mainly been studied in layers II/III and IV of barrel cortex to date and little is known of layer V EDP. Layer V contains a relatively heterogeneous population of neurons including intrinsic bursting (IB) and regular spiking (RS) cells as well as sublaminae Va and Vb. Previous studies have suggested that sub-types of layer V pyramidal cells show different levels of spine plasticity (Holtmaat et al., 2006). To test whether functional plasticity was also different between sub-types of pyramidal cells we performed single-row whisker deprivation lasting 0, 3 or 10 days in 6 week old rats and recorded both evoked and spontaneous activity with extracellular carbon fiber or intracellular sharp electrodes in layer Va and Vb of the deprived barrel columns under urethane anesthesia (1.5 g/kg, I.P. injection). We classified intracellularly-recorded neurons as RS and IB cells. We investigated whether some properties of the spontaneous activity could be used to reveal the intrinsic properties of the cells. Firstly, we did not find any obvious relationship between the timing of the spikes in a spontaneous train and the intrinsic properties of the cells. Secondly, we observed that the ratio between the amplitude of the second spike vs. the first spike in a pseudo-burst (inter-spike interval below 10 ms) is significantly smaller for IB cells (mean ± SD: 0.72 ± 0.12) than for RS cells (0.90 ± 0.10, Student’s unpaired t-test p<0.005). After 3 days and 10 days of deprivation, we observed trough extracellular recordings a depression of the deprived principal whisker and a potentation of the spared whiskers in both layers Va and Vb. We calculated the spike amplitude ratio only for the best discriminated pseudo-bursts during spontaneous activity. Following deprivation, we found a link between the shape of the receptive field and the spike amplitude ratio. For 3 days but not 0 days of deprivation, the response to stimulation of the principal whisker is positively correlated in layer Va (r2 = 0.19, P<0.01) and negatively correlated in layer Vb (r2 = 0.15, P<0.05) with the spike amplitude ratio. The depression is cell-type and sublaminae-dependent. In both layers Va and Vb, the response to the best spared whisker is positively correlated with the spike amplitude ratio after 10 days of deprivation (layer Va, r2 = 0.97, P<0.0005; layer Vb, r2 = 0.28, P<0.001) but not after 3 days. The potentiation is more pronounced for RS-like cells than for IB-like cells. Our results suggest that sensory deprivation differentially affects the subclasses of neurons in layer V of the barrel cortex. This observation could result either from differences in the input patterns or from different intrinsic mechanisms for inducing plasticity in different sublaminae and subtypes of layer V pyramidal neurons.